The present invention relates generally to tunable electronic devices and components and, more particularly, to voltage controlled oscillators incorporating tunable ferroelectric components.
Radio frequency bandwidth is a scarce resource that is highly valued and is becoming increasingly congested. Ever-increasing numbers of users are attempting to co-exist and to pass ever-increasing amounts of information through the finite amount of bandwidth that is available. The radio spectrum is divided into frequency bands that are allocated for specific uses. In the United States, for example, all FM radio stations transmit in the 88-108 MHz band and all AM radio stations transmit in the 535 kHz-1.7 MHz band. The frequency band around 900 MHz is reserved for wireless phone transmissions. A frequency band centered around 2.45 GHz has been set aside for the new Bluetooth technology. Hundreds of other wireless technologies have their own band of the radio spectrum set aside, from baby monitors to deep space communications.
Communications within a given frequency band occur on even more narrowly and precisely defined channels within that band. Hence, in virtually any wireless communication system or device, frequency agility is required and accurate frequency generation is of critical importance. Frequency generation is typically provided by an electronic oscillator. As is well known in the art, an electronic oscillator is a circuit that produces an output signal of a specific frequency, and consists generally of an amplifier having part of its output returned to the input by means of a feedback loop. A very simple electronic oscillator includes some combination of a capacitor with an inductor or other resonator.
The capacity for frequency channel selection and changing can be provided by a voltage controlled oscillator (xe2x80x9cVCOxe2x80x9d). In a VCO, a control voltage is applied to a voltage dependent capacitor, commonly referred to as a variable capacitance diode, varicap diode or varactor, in order to tune the VCO to a particular frequency. FIG. 1 illustrates a conventional varicap diode tuned oscillator resonant circuit 100. Circuit 100 includes varicap diode D1 and resonator L1 (L1 is an inductor or some other form of resonant transmission line device). Control voltage V1 is applied across varicap diode D1 via input resistance R1. V1 is a DC control voltage and is applied to tune the oscillator over a specified range. DC blocking capacitor C1 is interposed between varicap diode D1 and inductor L1, and DC blocking capacitor C2 is interposed between inductor L1 and an oscillator sustaining amplifier (not shown). Typically, the sustaining amplifier is a negative impedance generator. As is well known in the art, phase-locked loop (PLL) control circuitry will also typically be provided in conjunction with the VCO.
When a reverse voltage (V1) is applied to varicap diode D1, the insulation layer between the p-doped and n-doped regions of the semiconductor thickens. A depletion region that is essentially devoid of carriers forms in diode D1, and behaves as the dielectric of the capacitor. The depletion region increases as the reverse voltage across it increases, and since capacitance varies inversely as dielectric thickness, the junction capacitance decreases as the reverse voltage increases. The effect is similar to separating the two plates of a capacitor by a-larger distance, which decreases the capacitance. So, by varying the control voltage V1 the junction capacitance provided by varicap diode D1 can be varied. Varying the capacitance, in turn, changes the resonant frequency of inductor L1 and hence the frequency that will be amplified and output by circuit 100.
In recent years, VCO designers have been required to comply with significantly more demanding specifications Currently, only a handful of manufacturers world wide can economically produce VCOs that are suitable for use in high volume consumer communication devices. Two of the major hurdles faced in VCO design are (1) phase noise and (2) the inherent non-linear transfer function (applied voltage versus capacitance) of varicap diodes.
One critical parameter of oscillator performance is its single sideband phase noise, or simply xe2x80x9cphase noisexe2x80x9d. Phase noise affects the receiver""s ability to reject unwanted signals on nearby channels. It is the ratio of the output power divided by the noise power at a specified offset and is expressed in dBc/Hz. FIG. 2 is a graph showing the typical phase noise requirement for a 1 GHz VCO. As can be seen, at an offset of about 60 kHz a 1 GHz oscillator specifies a phase noise of about xe2x88x92120 dBc/Hz.
One of the main stumbling blocks to achieving this performance is the loaded Q of the oscillator circuit. The sustaining amplifier of the oscillator does not usually play a significant factor in phase noise determination due to the availability of low noise semiconductors that are specifically optimized for this purpose. The loaded Q of the resonator structure (L1) is typically the dominant factor in determining the overall phase noise performance. The loaded Q of the resonator is frequently limited by the series resistance of the varicap diode, which can be as much as several ohms.
The Q of a capacitor can be expressed by:
Q=Xc/Rs, where Xc is the reactance of the varicap diode given by Xc=1/(2xc2x7xcfx80xc2x7fxc2x7c),
and Rc is the effective series resistance of the varicap diode.
If a required capacitance of 5 pF at a frequency of 1.5 GHz is assumed, a reactance Xc of 21.22xcexa9 results. If it is further assumed that the effective series resistance Rs of the varicap diode is 0.5xcexa9, the resultant Q of the varicap diode is 42.44. Hence, reducing the effective series resistance will have a direct impact on the Q of the varicap diode and the loaded Q of the entire resonator structure.
Another critical parameter in oscillator performance is the linearity (or lack thereof) in the transfer function (applied voltage versus capacitance) of the varicap diode. FIG. 3 is a chart plotting the capacitance of a typical varicap diode versus a typical tuning voltage range for the diode in a mobile phone (0.3V to 2.7V). As can be seen, it is not a linear relation. Below 0.5V, unit voltage changes lead to much greater unit capacitance changes. Consequently, the MHz/volt frequency shift of the oscillator is not constant across the tuning range. This leads to compromise in the design of the PLL loop filter and thus overall noise performance.
Another problem associated with the use of varicap diodes is that, since it is a reverse-biased diode junction, it is important that the applied AC signal does not overcome the bias voltage and result in heavy forward conduction of the diode. If this occurs the Q of the resonator will be dramatically lowered and various oscillator parameters such as phase noise and general spectral purity will be seriously impacted. In an extreme case, the oscillator may fail to maintain a continuous oscillation and degenerate into parasitic uncontrolled burst oscillations.
In view of the above, there is a need for a voltage controlled oscillator that exhibits better phase noise performance and a more linear voltage/capacitance transfer function.
The present invention provides a voltage controlled oscillator that incorporates a tunable ferroelectric capacitor to provide better phase noise performance and a more linear voltage/capacitance transfer function.
Accordingly, in one embodiment of the invention, a voltage controlled oscillator is provided that has a resonant circuit for generating a tuning frequency. The resonant circuit comprises an inductive element and a ferroelectric capacitor having a variable capacitance. A control line is coupled to the ferroelectric capacitor for applying a control voltage to the capacitor to vary the capacitance which, in turn, varies the tuning frequency of the resonant circuit.
In another embodiment of the invention, a voltage controlled oscillator is provided. The oscillator includes a resonant circuit having a first variable ferroelectric capacitor to generate a signal having a variable resonant frequency, and an amplifier coupled to the resonant circuit to amplify the signal. A feedback loop coupled between the amplifier and the resonant circuit incorporates a second ferroelectric capacitor to control the amplitude and phase of a feedback signal.
Another embodiment of the invention comprises a band-switchable oscillator resonant circuit. The circuit has first and second ferroelectric capacitors and first and second control voltage lines to facilitate band switching.
The present invention also provides a method for band switching in a voltage controlled oscillator. First and second ferroelectric capacitors are provided, and first and second control voltages are applied to the first and second capacitors so that either the first capacitor or the second capacitor dominates the output frequency of the oscillator.
Other features, objects and implementations of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. All such additional features, objects and implementations are intended to be included within this description, to be within the scope of the invention and to be protected by the accompanying claims.